Process for recovering thorium and rare earth values



Dec. 1, 1964 B. J. LERNER 3,

PROCESS FOR RECOVERING 'THORIUM AND RARE EARTH VALUES Filed Feb, 9. 1960 IN VEN TOR. see/mew LEE/V51? BY JTTORNEY United States Patent 3,159,452 PROCESS FOR RECOVERING THQM AND RARE EARTH VALUES Bernard J. Lerner, Pittsburgh, Pa, assignor to Gulf Research 3: Development Company, Pittsburgh, Pa, a corporation of Delaware Filed Feb. 19, 1969, Ser. No. 9,882 3 Claims. (Cl. 23-145) This invention relates to the recovery of thorium and rare earth elements from ores containing the same.

The rare earth elements are those elements having atomic numbers 57'to 71 inclusive, and include lanthanum, cerium, praseodymium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutecium. The predominant constituent of the rare earths is cerium which often constitutes up to 50 percent of the totalrare earth content.

The commercial ores in which rare earth elements are found most abundantly are primarily monaz-ite and bastnasite. Monazite is an orthophosphate of the rare earth elements and thorium. Bastnasite is a cerium fluorocarbonate. The rare earth elements are also found, usually in relatively small amounts, in various other ores such as for example, euxenite, rnicrolite, lanthanite, lyndochite, and pyrochlore ores in which the rare earths are associated with niobium. Such rare earth ores usually contain high proportions of lime which term as employed herein refers to calcium carbonate alone and in conjunction withone or more of the group of calcium, barium, and strontium compounds usually as carbonates, silicates or oxides;

Generally, strong mineral acids such as sulfuric, are employed to open the ores to give access to the rare earth content. This procedure has the disadvantage particularly 'when treating calcareous ores in that the sulfuric acid reacts with the lime forming large amounts of sulfate salts such as calcium sulfate. This, of course, results in high acid consumption. Moreover, the sulfate salts precipitate to form a rather cementitious gel or paste which presents considerable difficulty in handling of the ores and which often prevents eflicient separation of the rare earth values therefrom. These disadvantages are, of course, more significant in the case of those ores which contain relatively high proportions of lime compared to the rare earth'v-alues.

The present invention provides a highly economical and efficient means for separating thorium and rare earth elements from ores and concentrates in which they are present. The process of the invention is particularly advantageous for recovering thoriumand rare earth elements from materials in which they are associated with large amounts of lime.

In accordance with the present invention, materials containing rare earth elements and/ or thorium are treated by leaching with an aqueous solution containing sulfur dioxide gas to produce a leach solution containing the rare earth elements. Lime which may be present in the material treated, reacts with the sulfur dioxide to produce carbon dioxide and calcium or other alkaline earth metal bisulfite compounds which are soluble in the leach solution. By this procedure, the formation of cementitious precipitates which tend to impair the separation of the rare earths is avoided.

The treatment of the rare earth-containing material so as to effect substantially complete recovery of the rare earth elementsis accomplished according to the present invention bytrea-ting an aqueous slurry of the rare earth ore, concentrate, or other material with sulfur dioxide gas. The sulfur dioxide gas can either be added to the ore slurry or to the aqueous solution before formation of the slurry. Sufiicient Water is em- 3,159,452 Patented Dec. 1, 1964 ployed in forming the slurry to give a pulp having a liquid to solid ratio of about 1:3 to 15:1. The ore should be in a finely divided state and preferably the particle size of the ore should be within the range of from about ,4 of an inch to ,5 of an inch. The treatment can be advantageously effected at atmospheric pressures and temperatures. The amount of sulfur dioxide required in the process to accomplish the desired results depends upon various factors including the quantity of rare earth elements, thorium, and other metals in the ore treated which may be solubilized during the leach process; Additional amounts of sulfur dioxide are required for reaction with the lime content of the ore. In all cases sufficient sulfur dioxide is utilized in the process to effect substantially complete solubilization of the desired rare earth compounds.

One particularly advantageous method for conducting the ore treatment involves forming an aqueous slurry of the rare earth-containing ore or concentrate in a closed reaction vessel and continuously introducing sulfur dioxide gas thereto at a relatively uniform rate while measuring the rate of generation of off-gas from the reactor. Introduction of the sulfiur dioxide gas is continued until the slurry does not consume further sulfur dioxide, that is, until the rate of discharge of gas from the reaction vessel is substantially equal to the rate at which sulfur dioxide is introduced thereto. The pH of the leach solution at this point is about 1.0 to 2.0. At this point reaction of the sulfur dioxide with the lime present in the ore and the desired rare earth elements is complete and introduction of sulfur dioxide is discontinued. During introduction of sulfur dioxide the slurry should be agitated to distribute the sulfur dioxide through the slurry and to increase mass transfer across the film of liquid adhering to the ore particles. The rate of introduction of sulfur dioxide into the ore slurry should not be so low as to require inordinately long treating times nor should the rate of introduction of sulfur dioxide be so high as to result in loss of sulfur dioxide from the reaction vessel without reaction therein. slurry at a rate of from about 0.1 to 1 pound of sulfur dioxide per hour per pound of initial ore charge. This operation can be made continuous by charging the aqueous ore slurry to a series of tanks, counter-current to the flow of sulfur dioxide gas, and monitoring the ottgas rate from each tank. In this manner, the ore treating operation can be conducted continuously with substantially complete utilization of sulfur dioxide.

Characteristically, when treating materials containing relatively high proportions of 'carbonates,-fiow of off-gas begins promptly upon introduction of the sulfur dioxide due to evolution of carbon dioxide as a result of reaction of the sulfur dioxide with the lime. When the reaction of sulfur dioxide with the carbonate content of the ore is substantially complete, the rate of off-gas generation falls sharply. The off-gas rate remains low for a period while sulfur dioxide is absorbed by the slurry and finally builds up rapidly until the off-gas is percent sulfur dioxide at the point where the off-gas rate is substantially the same as the rate of introduction of sulfur dioxide.

Upon completion of the sulfur dioxide leaching step, the leach liquor containing the recovered rare earth values is separated from the spent ore solids by filtering or centrifuging.

The sulfur dioxide leach liquor containing the desired rare earth values in the form of bisulfite compounds is then treated by a variety of suitable procedures for recovcry of the rare earths. To this end, according to a preferred embodiment of the invention, the rare earth bisulfites are converted to rare earth sulfates, following which thorium and the individual rare earths are separated by In practical operation, sulfur dioxide is fed to the a methods known in the art. Thus, the leach liquor solution can be treated with sulfuric acid to metathetically convert the rare earth bisulfites to the sulfates with the liberation of sulfur dioxide. Sufiicient sulfuric acid is employed to insure that substantially all of the rare earth bisulfites are converted to the sulfates. Calcium bisulfite which may be present in the leach liquor is converted to calcium sulfate by the action of the sulfuric acid. However, the rare earth sulfates are soluble and remain in Solution while the calcium sulfate precipitates. The rare earth sulfates can thus be separated from the precipitated calcium sulfate by decantation, centrifugation or filtration. After separation from the calcium sulfate, the solution containing the rare earth sulfate is then processed for separation of the individual rare earth metals by any suitable method known to the art, the methods used not being part of the present invention.

In an alternate method of operation, the liquor or iitrate resulting from the sulfur dioxide leach is subjected to aeration at ambient temperatures in the presence of a suitable oxidation catalyst such as cobaltic chloride or cupric sulfate or chloride. The aeration of the leach liquor in the presence of an oxidation catalyst results in the oxidation of the rare earth bisulfites with the formation of the rare earth sulfates. The air should be properly diffused and this may be accomplished with conventional types of gas diffusers. Aeration is continued for a period of about 1 to 6 hours.

Recovery of the rare earth values by conversion of the rare earth bisulfites to the sulfates by the procedures described is particularly advantageous in the processing of ores which contain relatively large amounts of lime. While the calcium in the lime is solubilized during the sulfur dioxide leach and reacts with the sulfuric acid in the leach liquor treating step, the calcium sulfate which is formed is insoluble and can be readily separated from the soluble rare earth sulfates which remain in solution. This procedure permits the rare earth values to be easily and elficiently separated from the calcium present in the ore.

In the case of ores containing very small quantities of metals, other than the desired rare earths which are solubilized by sulfur dioxide leaching, the rare earths can be recovered from the sulfur dioxide leach liquor by a procedure which involves steam stripping. Steam stripping of the sulfur dioxide leach liquor liberates sulfur dioxide from solution and converts the rare earth bisulfite compounds to rare earth sulfites. The rare earth sulfite compounds form a precipitate which is easily separated by filtration or other means, and which can be roasted at temperatures of about 300-600 F. to recover additional sulfur dioxide and to yield as a final product the rare earth oxides.

As a further variation in processing ores containing relatively small amounts of lime, the desired rare earths can be recovered from the sulfur dioxide leach liquor by an oxalate precipitation whereby the rare earth and thorium values are precipitated. For oxalate precipitation of the rare earths, the leach liquor should have a pH of from about 0.4 to 5, a condition which is normally achieved as a result of leaching with sulfur dioxide. The oxalic acid anions are preferably added in the form of oxalic acid with a saturated aqueous solution thereof (about percent) being preferred. The oxalic acid is used in amounts in excess of that stoichiometrically required for conversion of all of the thorium and the rare earths present. The solutions are thoroughly mixed by agitation and then allowed to stand for a few hours after which time the oxalate precipitates of the rare earth elements can be readily filtered from the solution.

In addition to the methods described in the preceding paragraphs, recovery of the rare earth values from the sulfur dioxide leach liquor can be accomplished by other means such as solvent extraction utilizing a solvent such as tri-cresylphosphate or the like, or by ion exchange processes and other methods known to the art. The invention contemplates the use of any recovery method which accomplishes the separation of rare earth metals from solutions in which they are present in the form of bisulfite compounds.

As illustrative embodiments of a preferred manner in which the invention may be practiced, the following examples are presented:

Example I Referring to the process illustrated in FIGURE 1, a bastnasite-type ore containing about 15 percent rate earth compounds, about 20 percent lime (calcium carbonate), about 7 percent iron and aluminum oxides and the remainder consisting primarily of barium sulfate and silica, is milled so that about percent of the particles pass a mesh screen. The finely'divided ore is introduced into slurry tank 1 and slurried with water. Sufiicient water is employed to produce a pulp having a liquid to solid weight ratio of about 4:1. This slurry of finely ground ore and water is forced by slurry pump 2 from tank 1 through conduit 3 into leach tank or digester 4. Digester 4 is a closed chamber with a draft tube 5 positioned concentrically within the outer walls of the digester. Since the purpose of a draft tube is to permit circulation of the contents of the chamber upward inside the tube and downward in the space between the outer wall of the draft tube and the inside wall of the chamber, the draft tube does not extend either to the bottom of the chamber or at its upper end, quite to the level at which liquid will be maintained in the digester. In this preferred manner of conducting the ore treatment, it is desired to measure the rate of gas generation in the digester and for this purpose digester 4 is fitted with a pressure gauge 6 and a flow meter 7 to measure the rate at which off-gas leaves the digester through conduit 8. It is always desirable to operate with as small a gas space as possible above the liquid in the digester 4 and to maintain a substantially uniform pressure therein, as this will permit maximum accuracy in determining the current rate of generation of off-gas. With the aid of pump 9, sulfur dioxide is introduced into digester 4 from conduit ltl through sparge 11. It is also desired to measure the rate at which sulfur dioxide is introduced into the digester 4 through conduit 10 and sparge 11 and for this purpose a flow meter 12 and pressure gauge 13 are placed in conduit 16 close to the point of discharge into digester 4. It is desirable, at least during portions of the process, to have positive means for circulation of the slurry in digester 4 and for this purpose provision is made to remove gas from the top of digester 4 through conduit 14 and return it to the bottom of the digester by means of blower l5, conduit 16, and sparge 17. Mechanical stirring means such as paddles, propellers and regenerating pumps and so forth, are equally satisfactory.

A sight gauge 18 is provided to permit observation of the level of the material in digester 4. The digester is also provided with a draw-off line 19 fitted with a screened intake 2t A shield 21 positioned above screened intake 20 and at some distance therefrom helps to keep ore particles from being withdrawn through draw-off line 19. A sampling connection 22 provides for the withdrawal of test samples from digester 4.

When treating ores containing appreciable quantities of carbonates once the feed of the sulfur dioxide to digester 4 is commenced there will be almost immediate generation of gas and the flow then proceeds to increase to about its maximum. The supply of sulfur dioxide is maintained uniform and the pressure on gauge 6 is maintained uniformly at a low figure. The rate of generation of off-gas as measured at the outlet of the digester in conduit 8 will continue at an elevated and relatively uniform rate until the carbonates are practically completely removed, whereupon, the rate of generation of off-gas will fall away rapidly. This does not mean that the rate of generation of off-gas will 'be precisely uniform particularly 1f the rate of supply of sulfur dioxide is varied, but it does mean that the ratio of the rate of generation of gas to the rate of supply of sulfur dioxide will be about the same until substantially all of the carbonates in the ore have reacted. As the supply of sulfur dioxide to digester 4 is continued at a uniform rate, the rate of discharge of off-gas will rapidly build up to a point where it is substantially the same as the rate of input. At this point, which may be designated the sulfur dioxide saturation point, equilibrium exists between the sulfur dioxide and sulfurous acid and introduction of sulfur dioxide is discontinued inasmuch as reaction of the sulfur dioxide with substantially all of the lime and soluble metal values of the ore is completed. At this time, the supply of sulfur dioxide is generally discontinued and the digested slurry leaves the bottom of digester 4 through conduit 23 and is forced by slurry pump 24 through conduit 25 to filter 26. Water is supplied as a filter wash liquid through conduit 28. Solids separated by filter 26 are discharged therefrom at 27 and the filtrate from filter 26 containing the soluble rare earth compounds and other metals such as calcium, iron and aluminum in the form of bisulfite compounds, is discharged therefrom through conduit 29 into reactor 30. Sulfuric acid which is supplied to the reactor 30 through conduit 31 reacts with and converts the calcium and rare earth bisulfites to the corresponding sulfates. After the reaction of the sulfuric acid with the bisulfite solution in reactor 30 is complete, the reaction products are conveyed therefrom through conduit 32 and forced by pump 33 through conduit 34 to filter 35. The solution containing the soluble rare earth sulfates is removed from filter 35 through conduit 36 and is then subjected to further processing as desired. Solid calcium sulfate is discharged from filter 35 and caught in bin 37.

Example 11 Approximately 100 grams of a rare earth-containing ore obtained from the northeast shore of Lake Nemegosenda in Ontario, Canada, were slurried with water in a two-liter autoclave. Approximately 800 milliliters of water were employed to give a water to ore weight ratio of about 8:1. Sulfur dioxide was supplied to the autoclave at a relatively uniform rate of about 32.5 grams per hour for a period of about 1.5 hours. At the end of this time, the off-gas rate had become equal to the input rate indicating that the sulfur dioxide was no longer being consumed. Introduction of sulfur dioxide was discontinued and the aqueous slurry was then filtered on a Biichner funnel and the filter cake washed at room temperature with four 100 milliliter portions of water to reduce the filtrate pH to 5. The rare earths were precipitated from the filtrate by the addition of an 8 percent oxalic acid solution. Analysis of the charge ore and residue resulting from treatment with sulfur dioxide using the X-ray fluorescent method was as follows:

Percent in Ore Element Percent in Residue Result- Charge Ore ing from 80;

Treatment ing rare earth-containing materials with sulfur dioxide in accordance with the present invention. Thus, for example, rare earth-containing materials can in some cases be continuously treated by percolation with a solution containing sulfur dioxide.

It will be appreciated from the foregoing that the process of the invention provides a highly advantageous and inexpensive method for recovering rare earth values from materials containing the same. The process is particularly advantageous for recovering rare earth values from high-lime or high silicate ores in that the lime content of the ore is effectively neutralized without the formation of cementitious precipitates as are encountered with the use of sulfuric acid and which present serious problems in further processing of the ore. Other advantages of the process are that the process provides highly efficient separation of the rare earths with maximum economy in respect to the reagents and equipment required. Successful operation of the process at atmospheric temperature and pressure is a further advantage.

Those modifications and equivalents which fall Within the spirit of the invention and the scope of the appended claims are to be considered part of the invention.

I claim:

1. A process of recovering thorium and rare earth values from ores containing the same together with substantial amounts of calcium which comprises: treating the ore in finely divided condition with an aqueous solution containing sulfur dioxide to obtain an aqueous liquid reaction product having dissolved therein calcium, thorium and rare earth metal bisulfites, selectively precipitating calcium from said aqueous liquid reaction product by converting the said thorium, calcium and rare earth metal bisulfite compounds to the respective metal sulfate compounds, separating the precipitated calcium sulfate from the aqueous liquid reaction product, and thereafter treating the remaining liquid reaction product to separate the thorium and rare earth values therefrom.

2. A process of recovering thorium and rare earth values from ores containing the same together with appreciable amounts of calcium which comprises: treating the ore in finely divided condition with an aqueous solution containing sulfur dioxide to obtain an aqueous liquid reaction product having dissolved therein thorium, calcium and rare earth metal disulfites, treating said liquid reaction product with sulfuric acid to convert the thorium, calcium and rare earth metal bisulfites to the respective metal sulfates, and then separating the liquid reaction product containing dissolved thorium and rare earth sulfates from the insoluble solid calcium sulfate.

3. A process of recovering thorium and rare earth values from ores containing the same together with appreciable amounts of calcium which comprises: treating the ore in finely divided condition with an aqueous solution containing sulfur dioxide to obtain an aqueous liquid reaction product having dissolved therein thorium, calcium and rare earth metal bisulfites, aerating said liquid reaction product in the presence of an oxidation catalyst effective to convert the thorium, calcium and rare earth metal bisulfites to the respective metal sulfates, and then separating the liquid reaction product containing dissolved thorium and rare earth sulfates from the insoluble solid calcium sulfate.

References Cited in the file of this patent UNITED STATES PATENTS 702,582 Neill et al. June 17, 1902 890,584 Fleck et al June 9, 1908 1,095,377 Burfeind May 5, 1914 2,128,027 Clark Aug. 23, 1938 2,176,610 Stamberg Oct. 17, 1939 2,425,573 Soddy Aug. 12, 1947 2,849,286 Welt et al Aug. 26, 1958 2,863,716 Thunaes Dec. 9, 1958 

1. A PROCESS OF RECOVERING THORIUM AND RARE EARTH VALUES FROM ORES CONTAINING THE SAME TOGETHER WITH SUBSTANTIAL AMOUNTS OF CALCIUM WHICH COMPRISES: TREATING THE ORE IN FINELY DIVIDED CONDITION WITH AN AQUEOUS SOLUTION CONTAINING SULFUR DIOXIDE TO OBTAIN AN AQUEOUS LIQUID REACTION PRODUCT HAVING DISSOLVED THEREIN CALCIUM, THORIUM AND RARE EARTH METEL BISULFITES, SELECTIVELY PRECIPITATING CALCIUM FROM SAID AQUEOUS LIQUID REACTION PRODUCT BY CONVERTING THE SAID THORIUM, CALCIUM AND RARE EARTH METAL BISULFITE COMPOUNDS TO THE RESPECTIVE METAL SULFATE COMPOUNDS, SEPARATING THE PRECIPITATED CALCIUM SULFATE FROM THE AQUEOUS LIQUID REACTION PRODUCT, AND THEREAFTER TREATING THE REMAINING LIQUID REACTION PRODUCT TO SEPARATE THE THORIUM AND RARE EARTH VALUES THEREFROM. 